The mixing of molten plastic with additives including colorants is an important step prior to extruding the molten plastic into an extruded shape by means of a die or prior to injecting molten plastic into a mold during an injection molding process. Typically, plastic pellets and various additives are fed into the feed of an extrusion machine which typically includes a barrel and an extrusion screw which rotates within the barrel. Typically, the barrel is a precise cylindrical bore having heated walls. The screw is typically an elongated auger like member having a generally cylindrical screw root and a continuous helical ridge or “flight” which projects from the body. The helical flight presents a narrow (typically about 1 cm in width), continuous flat surface which is preferably closely offset from the inside wall of the barrel. Preferably, the flight is configured so that the extrudate does not flow between the flight and the inside wall of the barrel but, rather, is “wiped” from the inside wall. The extrudate is typically a combination of plastic pellets, partially melted plastic pellets, molten plastic and possibly additives. The proportions of unmelted plastic and molten plastic vary depending on whether the extrudate is near the intake or near the outlet. A channel is defined between the turns of the flight. This channel conveys the extrudate down the length of the barrel as the extrusion screw turns within the barrel. In most cases, the extrusion screw includes a feed section in which the depth of the channel is generally constant and relatively deep (on the order of 15 mm for a 100 mm diameter screw), a transition section in which the depth of the channel decreases and a first metering section wherein the depth of the channel is reduced (on the order of 7 mm for a 100 mm diameter screw) and constant. The depth of such a channel is known in the art as the “root depth”. Typically, the inside wall of the barrel includes a heating band for melting the plastic pellets into a highly viscous molten plastic material. In order to accomplish thorough mixing, extrusion screws often include a mixing section. The purpose of a mixing section is to mix the molten plastic and additives as much as possible prior to a second metering section which forces the extrudate either into a die or an injection mold